Flexible pipes of the present type are well known in the art in particular for offshore transportation of fluids. Such pipes comprise usually an inner liner also often called an innermost sealing sheath or an inner sheath, which forms a barrier against the outflow of the fluid which is conveyed through the pipe, and one or more armoring layers on the outer side of the inner liner (outer armoring layer(s)). An outer sheath may be provided with the object of forming a barrier against the ingress of fluids from the pipe surroundings to the armor layers.
Typical unbonded flexible pipes are e.g. disclosed in WO0161232A1, U.S. Pat. No. 6,123,114 and U.S. Pat. No. 6,085,799.
In order to have sufficient strength, in particular to prevent the collapse of the innermost sealing sheath, the flexible pipe often comprise an armor layer located inside the space defined by the innermost sealing sheath. Such carcass layer or layers are normally referred to as a carcass.
The flexible pipes are often unbonded pipes. The term “unbonded” means in this text that at least two of the layers including the armoring layers and polymer layers are not bonded to each other. In practice the known pipe normally comprises at least two armoring layers located outside the innermost sealing sheath. The armoring layers are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation. The outer armor layer or layers located outside the innermost sealing sheath are normally referred to as pressure armor, tensile armor or balanced pressure and tensile armor.
The above-mentioned type of flexible pipes is used, among other things, for off shore as well as some on-shore applications for the transport of fluids and gases. Flexible pipes can e.g. be used for the transportation of fluids where very high or varying water pressures exist along the longitudinal axis of the pipe, such as riser pipes which extend from the seabed up to an installation on or near the surface of the sea, pipes for transportation of liquid and gases between installations, pipes which are located at great depths on the seabed, or between installations near the surface of the sea.
In traditional flexible pipes, such as steel based flexible pipes i.e. where the armoring layers are mainly of steel, the armoring layers are in the form of helically wound profiles or wires, where the individual layers may be wound with different winding angles relative to the pipe axis in order to take up the forces caused by internal and external pressure as well as forces acting at the ends of the pipe and shear forces from the surrounding water. The carcass is typically wound from preformed or folded stainless steel strips.
A pipe of the above type will for many applications need to fulfill a number of requirements. First of all the pipe should have high mechanical strength to withstand the enormous forces it will be subjected to during transportation, laying down and in operation. The internal pressure (from inside of the pipe and outwards) and the external pressure (from outside of the pipe) are very high and may vary considerably along the length of the pipe. If the pipe resistance against the internal pressure is too low the internal pressure may ultimately result in that the pipe is damaged e.g. by upheaval buckling and/or burst of the flexible pipe. If the pipe resistance against the external pressure is too low the external pressure may ultimately result in deformation and/or Birdcaging of the flexible pipe and/or collapse of the innermost sealing sheath which is acting as the primary barrier towards outflow of a fluid transported in the flexible pipe. Simultaneously the flexible pipe may be subjected to highly corrosive fluids and chemical resistance may be needed. Furthermore, it is important that the weight of the pipe is kept sufficiently low as a too high weight may render certain use impossible. Furthermore the weight may also influence production and deployment costs.
In general flexible pipes are expected to have a lifetime of 20 years in operation.
The object of the invention is to provide a flexible pipe which pipe can be provided in long length with a desired strength and strength profile along its length and which pipe can be manufactured in a cost effective manner.
The present invention provides a novel flexible pipe and a method for its production which meet this object. The flexible pipe of the invention and embodiments thereof has shown to have a large number of advantages which will be clear from the following description.